An operating cycle of an internal combustion engine, as is well known in the art, consists of four phases in the 4-stroke Otto cycle correspond to respective piston strokes. These four stages comprises an intake phase for the aspiration of an explosive air/fuel mixture, a compression and ignition phase, an expansion or power phase and an exhaust phase. These internal combustion engines employ poppet type valves which require valve operating trains including valve springs, camshaft, etc. in order to convert the rotary motion of the engine into the linear movement required by the poppet valves. These poppet valves are normally opened by movement mechanically inwardly of a cylinder in which they are placed by means of a rocker arm actuated by a push rod which in turn has been actuated by hydraulic lifters or the like driven from a camshaft in synchronism with the operation of the engine. Valve return has usually been by spring means. While a cam in head engine eliminates the push rods that are otherwise required, the cam mechanism does include levers and springs for maintaining the valves in a closed position.
Conventional poppet valves have various problems associated with them. A conventional poppet valve engine requires considerable power to overcome the resistance to opening the valves against cylinder pressure. The application of the necessary power to open the valves produces wear in the valve train. Further, the members of the valve train are reciprocating. Thus power is dissipated in overcoming the inertia of the members in changing their direction. Such valve structure also requires additional hood height and is inefficient at high speeds. Further, since the valve in the train are constantly exposed to the high temperature of the ignited fuel in the cylinders, burning of the valves as sustained high speed operation is possible.
The timing of the opening of the intake and exhaust valve of an internal combustion engine equipped with conventional poppet valves is inflexible once established by the design of the camshaft. The desire of an engine for an intake charge, however, is different at high RPM and high load than it is at low speed and light load, or an idle and the effect of gas momentum at these dissimilar operating modes has a significant effect on performance, fuel economy and emissions.
At high engine speeds and moderate to heavy loads, a lengthy intake valve opening duration is required to permit efficient breathing and maximum power. Early opening of the valve during the intake stroke increases the length of time the valve is open during the early part of the stroke and late closing allows the charge momentum to continue filling the cylinder even though the piston is moving upward on the compression stroke. During high load operation the exhaust gasses exit the cylinder with such intensity that a relative vacuum can occur at the end of the exhaust stroke. An early intake valve opening can then be advantageous in obtaining increased volumetric efficiency.
During low engine speed and light load operation, the situation changes and the pressure of the exhaust gasses in the cylinder exceeds the pressure in the intake manifold so that early valve opening of the valve results in exhaust gasses entering the intake system, diluting the fresh mixture and reducing combustion efficiency. This is particularly significant an idle since the fuel system must compensate for this dilution with an extra rich mixture which increases fuel consumption and also the probability of increased emissions. Late closing of the intake valve at low to medium speeds and light to medium loads, where charge momentum does not at least counteract the piston's upwards push on the intake charge, results in already inducted intake charge being pushed back into the intake system and reduces the compression of the intake charge and the engine efficiency.
Inflexible valve timing, therefore, forces the engine designer to compromise in areas of performance, fuel economy and emissions since these areas are linked to and partially dependent on valve timing and improvements in one area usually result in deterioration in at least one of the others.
Engines incorporating rotary valves have proven superior in certain respects in that they can be made with larger valve openings and are not limited by restrictions imposed by camshaft configurations, such as the necessary rise and fall times of the poppet valve operating cams. Also, such rotary valve engines are basically simpler in that they eliminate the need for valve operating trains.
Although rotary valves have proven superior in certain respects, as mentioned above, the typical rotary valve engine suffers from the problems of fixed valve timing due to the design of the head ports and circumferential valve body openings. Accordingly, what is required is a design of a rotary valve system which also incorporates the ability to advance or retard the valve opening timing to better accommodate the various operating requirements of the engine.